The invention relates to a process for treating process water from the paper-making industry, in which at least one anaerobic purification and an aeration step are used.
Within the papermaking industry, there is an increasing trend towards reducing water consumption. There are various reasons for this, such as reducing the consumption of fresh water, reducing the amounts of waste water and limiting heat losses.
A drawback of reduced water consumption is that the concentrations of contaminants increase. To prevent problems with substantially closed circuits, it is increasingly common to use circuit purification. A circuit purification of this nature preferably comprises a biological purification which is used to remove dissolved organic substances, if necessary, followed by a sand filter for removing excess suspended matter.
Prior to the purification, the process water is cooled from 55xc2x0 C. to 35xc2x0 C., since it has hitherto been assumed that the bacteria thrive most successfully at this temperature. These mesophilic bacteria are unable to survive a temperature of higher than 44xc2x0 C.,
A purification of this nature is used at Zxc3xclpich Papier in Germany. The process comprises an anaerobic purification step, followed by an aerobic purification step and a sand filtration step. The aerobic purification comprises an activated sludge process, comprising aerated basins containing biomass followed by secondary settling tanks, in which the biomass is separated off and is returned to the aerated basins.
The purification process has been simplified at Kappa Graphic Board in Hoogezand, the Netherlands, where a biomass has been grown which achieves a high level of activity at a temperature of approx. 55xc2x0 C. As a result, the cooling can be dispensed with and the higher activity of the thermophilic bacteria even allows the size of the reactors to be reduced.
To make paper, in principle 200 liters of water is required per kilogram of paper produced. Circulation processes generally reduce this amount to less than 20 l/kg for white paper grades and less than 10 l/kg or often less than 5 l/kg for packaging paper grades.
Obviously, quality plays an important role in the reuse of the water. If all the surplus water could be recovered to drinking water quality, it could all be reused without problems and there would be no waste water. However, to meet this target the costs of purification are extremely high, since large quantities of water would have to be treated using membrane installations (UF and RO).
The invention relates to the use of various purification steps in succession and, if appropriate, in parallel, resulting in different qualities of purified effluent which can be reutilised at different points in a papermaking factory. For example, it is possible to supply a quantity of water which has passed through an anaerobic purification and an aerobic recovery step but from which suspended materials have not yet been removed. This water is in principle suitable as make-up water or dilution water at those areas in the factory where suspended material does not cause problems (for example in the pulper). Some of this water could undergo an additional treatment in which the suspended material is removed, for example by means of flotation or sedimentation. This water is suitable for use where amounts of suspended solids (SS) of up to 100 mg/l do not cause problems, for example for certain sprays. A further increase in quality can be achieved by positioning a sand filter downstream of the flotation or settling units. The result is water with an SS less than 10 mg/l, which is suitable for the finer sprays.
In the process of the invention, the solids separation step comprises a flotation and/or sedimentation and/or sand filtration and/or membrane filtration, preferably more than one of these separation steps. These different separation steps can be arranged in line, optionally with part of the each effluent being fed to the next separation step, and part being directly returned to specific points in the paper-making process, in accordance with the required water quality at that specific point.
The anaerobic treatment and/or the aerobic treatment, preferably both, may be incorporated in two or more different treatment lines. Preferably, two treated water streams have a volumetric ratio of 1:9 to 9:1. Suitably one of two treated streams additionally undergoes a sand filtration and/or membrane filtration, before being returned to the paper-making process Preferably at least one of the aeration steps has a hydraulic residence time of 1 to 8 hours, especially 2 to 6 hours. The anaerobic and/or the aerobic treatment may be partly or wholly carried out under mesophilic or thermophilic conditions, in particular at a temperature of 30 to 60xc2x0 C., preferably 40 to 60xc2x0 C. Advantageously the anaerobic purification step and/or the aeration step may be carried out in high tanks with a height of 16 to 24 m and 8 to 16 m, respectively. Aeration is carried out using conventional means, for example using injectors.
Finally, some of the biologically purified water can also be purified further with the aid of a UF membrane in order to supply a quantity of high-quality water which is completely free of suspended solids and in which the larger molecules (relating to the colour) have been removed. This water is suitable, for example, for use in high-pressure cleaners, edge trimmers and for making up chemicals. In this way, all the required qualities are provided, while the additional purification steps are kept as small as possible, in order to limit investment costs.
The use of the higher water temperature causes a higher biological activity and also promotes the flow through the membrane, so that the membrane surface area can be smaller. Furthermore, it has been found that the results of a UF treatment are considerably improved if this treatment is preceded by an anaerobic purification step, through which the smaller molecules have already been removed.